Synaptic terminals are subject to structural alteration in response to certain biological events. The objectives of this study are: a) analysis of the sequence of morphological events seen in terminals during development and degeneration; b) correlation of these changes with biochemical characteristics of the terminals in their different forms; c) an enhanced understanding of the morphological bases for physiological events in the auditory nuclei to be studied and of the information processing capabilities of these nuclei. Terminals in the medial superior olivary nucleus (MSO) have been chosen for study because the structural and synaptic organization of this nucleus are well understood and especially advantageous for the experiments planned. Terminals in other brain stem auditory nuclei will also be considered because of the features (including the presence of an intercellular substance) common to some classes of auditory terminals and the distribution of collaterals of auditory axons to these nuclei. Normal kittens, cats with anterior ventral cochlear nucleus lesions and inbred mice with genetically controlled auditory anomalies (especially involving congenital and post-natal hearing loss) will be used. They will be studied by electron microscopy of aldehyde perfusion and immersion material and light microscopy of Golgi, Nissl and reduced silver material. Horseradish peroxidase techniques will be used to trace recurrent collaterals in the MSO in order to identify different terminal types on a single neuron. Light microscopic autoradiographic methods will be used on fresh tissue slices to assay for transmitter related uptake systems. EM autoradiography with a physical developer will be used to study relationships between terminals and their internal and surrounding structures. Additional histochemical methods will be used as applicable.